1. Field of the Invention
The present invention relates to a method of driving a piezoelectric transformer used in a high-voltage generator, and to a power source apparatus employing the piezoelectric transformer.
2. Description of the Related Art
FIG. 16 illustrates a structure of a Rosen piezoelectric transformer which is one of typical piezoelectric transformers. Favorably, such the piezoelectric transformers can be sized smaller in the dimensions than electromagnetic transformers as is nonflammable and free from noises due to electromagnetic induction.
As shown in FIG. 16, the piezoelectric transformer comprises a low-impedance section 301 and a high-impedance section 302. The low-impedance section 301 acts as an input section when used for stepping up. The section 301 is polarized along the thickness direction, and has two electrodes 303u and 303d provided on both principle faces of the thickness direction. The high-impedance section 302 acts as an output section when used for stepping up. The section 302 is polarized along the long direction, and has an electrode 304 provided on a face of the long direction.
FIG. 17 shows a frequency response of the above described piezoelectric transformer. Recently the piezoelectric transformer has been used as a power source for a cold cathode ray tube because it has characteristics in that, the greater the load, the higher the step-up ratio increases (curve P1) and the smaller the load, the lower the step-up ratio decreases (curve P2).
The piezoelectric transformer may be driven by a separately-excited oscillator circuit which is provided with an external oscillator. FIG. 18 is a block diagram of a conventional drive circuit with a separately-excited oscillation method employing the Rosen-type piezoelectric transformer.
As shown in FIG. 18, a variable oscillator circuit 221 generates an alternating-current drive signal of which frequency is close to the resonant frequency of a piezoelectric transformer 110. The output signal of the variable oscillator circuit 221 contains other alternating-current signal components than the driving frequency signal. Those signal components generate heat or a loss in the piezoelectric transformer 110. To reduce the loss in the piezoelectric transformer 110, the output signal is shaped to substantially a sine wave by a waveform shaping circuit 224. The waveform shaping circuit 224 may simply be a band-pass filter for reducing harmonic components. The output of the waveform shaping circuit 224 is then amplified by a drive circuit 225 to a current level or a voltage level enough to actuate the piezoelectric transformer 110. The drive circuit 225 may comprise only a normal amplifier circuit composed of transistors, or a combination of an amplifier circuit and a step-up transformer. The output of the drive circuit 225 is stepped up by the piezoelectric transformer 110, applied to a load such as a cold cathode fluorescent lamp 108 to light on.
The piezoelectric transformer 110 may be varied in the resonant frequency depending on the ambient conditions including the temperature and the load. Therefore when the piezoelectric transformer 110 is driven at a constant frequency by the circuit shown in FIG. 18, relative relation between the resonant frequency and the driving frequency will change. More specifically, in case that the driving frequency is largely differentiated from the resonant frequency of the piezoelectric transformer 110, the voltage step-up ratio of the piezoelectric transformer 110 will decline significantly thus lowering the output voltage. As a result, the cold cathode fluorescent lamp 108 as a load may be supplied with insufficient current and fail to provide a desired level of luminance.
FIG. 19 is a block diagram of another conventional circuit for driving the piezoelectric transformer different from the circuit shown in FIG. 16 which can prevail over a change in the resonant frequency of the piezoelectric transformer 110. The cold cathode fluorescent lamp 108 serving as a load in the circuit is connected in series with a feedback resistor 109 having a small resistance. The feedback resistor 109 detects a current across the cold cathode fluorescent lamp 108. A resultant voltage across the feedback resistor 109 which is proportional to the current flowing through the cold cathode fluorescent lamp 108 is fed into a current detector circuit 232. An output of the current detector circuit 232 is applied to an oscillation control circuit 214. The oscillation control circuit 214 in turn controls the frequency of the output of the variable oscillator circuit 221 so that the voltage across the feedback resistor 109 or the current across the cold cathode fluorescent lamp 108 can be constant. This control permits the cold cathode fluorescent lamp 108 to light up at substantially a uniform level of luminance.
FIG. 20 is a block diagram of another modification of the conventional circuit for driving the piezoelectric transformer shown in FIG. 16 which can prevail over a change in the resonant frequency of the piezoelectric transformer. In this modification, the current across the cold cathode fluorescent lamp 108 is detected by the feedback resistor 109. When the resonant frequency of the piezoelectric transformer 110 is varied with a change in the load or the ambient conditions, the current across the cold cathode fluorescent lamp 108 may change. The voltage across the feedback resistor 109 which is proportional to the current across the cold cathode fluorescent lamp 108 is fed into the current detector circuit 232. The output of the current detector circuit 232 is then fed into a pulse width control circuit 223. The pulse width control circuit 223 in turns generates and delivers a control signal to a pulse width modifying circuit 222 so that the voltage across the feedback resistor 109 or the current across the cold cathode fluorescent lamp 108 can be constant. Upon receiving the control signal, the pulse width modifying circuit 222 adjusts the pulse width of the output signal to determine the amplitude of the voltage applied to the cold cathode fluorescent lamp 108. This control permits the cold cathode fluorescent lamp 108 to light up at substantially a uniform level of luminance.
As described above, the conventional drive circuit using the piezoelectric transformer controls the driving frequency of the piezoelectric transformer to keep a level of output current flowing through the load connected to the piezoelectric transformer constant. That is, to increase the output current, the driving frequency is kept away from the resonant frequency. In the conventional circuit, however, once the source voltage is declined, it becomes impossible to flow a sufficient level of current through the piezoelectric transformer even though the driving frequency is forced to match the resonant frequency. Thus, a desired level of the output current can not be provided. In reverse, when the source voltage is increased, the driving frequency is differentiated from the resonant frequency of the piezoelectric transformer hence lowering the driving efficiency. Also, in case that the load to the piezoelectric transformer is largely changed, simultaneously, the output current of the piezoelectric transformer can hardly be controlled to a specified value, and the driving frequency may be differentiated from the resonant frequency of the piezoelectric transformer, hence lowering largely the driving efficiency.
There are some techniques to modify the output voltage of the drive circuit without changing the driving frequency when the source voltage is varied or the load to the piezoelectric transformer is varied. One of the most known techniques is to modify the pulse width of the output voltage of the drive circuit. In this case, the narrower the pulse width or the smaller the duty, the greater harmonic components other than the basic driving frequencies will increase. As the harmonic components turn to thermal loss in the piezoelectric transformer, the driving efficiency and the operational reliability will be declined largely.
For solving the above drawbacks, a method is disclosed in Japanese Patent Laid-open Publication No. 9-135573. The method detects the relation between the resonant frequency and the driving frequency of the piezoelectric transformer through measuring a difference in the phase of current or voltage between the input and the output, and performs the frequency control while the frequency is within a predetermined range, or modifies the input power so as to keep a constant level of current across the cold cathode fluorescent lamp when the frequency is at the maximum or minimum limit of the range. However, the phase of the voltage or current of the input or output of the piezoelectric transformer is varied depending on the load. It is hence necessary for detecting the relation between the resonant frequency and the driving frequency of the piezoelectric transformer to perform adjustment depending on the inverter and the piezoelectric transformer.
The present invention is directed to eliminate the above drawbacks and its object is to provide a method of driving a piezoelectric transformer and a power source apparatus which can keep normal operation even though the source voltage or the load to the piezoelectric transformer changes largely, thus providing the high driving efficiency, the high operational reliability and the high durability in the piezoelectric transformer.
In a first aspect of the invention, provided is a method of driving a piezoelectric transformer which has a primary electrode and a secondary electrode, and which steps up a voltage input at the primary electrode with a step-up ratio which varies depending on a frequency according to a piezoelectric effect and outputs the stepped-up voltage from the secondary electrode. The method comprises detecting a linear differential value of the step-up ratio of the piezoelectric transformer with respect to the frequency, and controlling the driving frequency for the piezoelectric transformer according to the detected linear differential.
In a second aspect of the invention, provided is a method of driving a piezoelectric transformer which has a primary electrode, a secondary electrode and a third electrode and which steps up a voltage input at the primary electrode with a step-up ratio which varies depending on a frequency by a piezoelectric effect, and outputs the stepped up voltage from the secondary electrode and the third electrode. The method comprises detecting a linear differential value of a voltage ratio between the voltage input at the primary electrode and the voltage output from the third electrode to the frequency, and controlling the driving frequency for the piezoelectric transformer according to the detected linear differential value of the voltage ratio.
In a third aspect of the invention, provided is a method of driving a piezoelectric transformer which has a primary electrode, a secondary electrode and a third electrode, and which steps up a voltage input at the primary electrode with a step-up ratio which varies depending on a frequency by a piezoelectric effect, and outputs the stepped up voltage from the secondary electrode and the third electrode. The method comprises detecting a phase difference between the voltage input at the primary electrode and the voltage output from the third electrode, and controlling the driving frequency for the piezoelectric transformer according to the detected phase difference.
In a fourth aspect of the invention, a power source apparatus comprises a piezoelectric transformer with a primary electrode and a second electrode for stepping up a voltage input at the primary electrode by the piezoelectric effect to output the stepped-up voltage from the secondary electrode, a driving section for driving the piezoelectric transformer at a desired voltage and at a desired frequency, a current detecting section for measuring a current across a load which is driven by the voltage output from the secondary electrode of the piezoelectric transformer, a step-up ratio differential detecting section for determining a linear differential value of the step-up ratio of the piezoelectric transformer with respect to the frequency, and a control section for controlling the driving frequency and the driving voltage for the piezoelectric transformer according to the current across the load detected by the current detecting section and the linear differential value determined by the step-up ratio differential detecting section.
In a fifth aspect of the invention, a power source apparatus comprises a piezoelectric transformer with a primary electrode, a secondary electrode and a third electrode for stepping up a voltage input at the primary electrode by the piezoelectric effect to output the stepped up voltage from the secondary electrode and the third electrode, a driving section for driving the piezoelectric transformer at a desired voltage and at a desired level frequency, a current detecting section for detecting a current across a load which is driven by the voltage output from the secondary electrode of the piezoelectric transformer, a step-up ratio differential detecting section for determining a linear differential value of a voltage ratio between the driving voltage for the piezoelectric transformer and the voltage output from the third electrode, to the frequency, and a control section for controlling the driving frequency and the driving voltage for the piezoelectric transformer based on the current detected by the current detecting section and the linear differential value of the step-up ratio determined by the step-up ratio detecting section so that the current across the load is at a predetermined level.
In a sixth aspect of the invention, a power source apparatus comprises a piezoelectric transformer with a primary electrode, a secondary electrode and a third electrode for stepping up a voltage input at the primary electrode by the piezoelectric effect to output the stepped up voltage from the secondary electrode and the third electrode, a driving section for driving the piezoelectric transformer at a desired voltage and at a desired frequency, a current detecting section for detecting a current across a load which is driven by a voltage output from the secondary electrode of the piezoelectric transformer, a phase difference detecting section for detecting a phase difference between the voltage input to the primary electrode and the voltage output from the third electrode in the piezoelectric transformer, and a control section for controlling the driving frequency and driving voltage for the piezoelectric transformer based on the current detected by the current detecting section and the phase difference detected by the phase difference detecting section so that the current across the load is at a predetermined level.